A) Field of the Invention
This invention relates to a semiconductor light emitting element array and an automotive lighting using the semiconductor light emitting element arrays.
B) Description of the Related Art
High power is required for a light emitting diode (LED) element for use in headlamps for vehicles, illuminations or the likes. If a size of the element is simply enlarged, a driving electric current becomes too large and it becomes difficult to flow an electric current uniformly in the element. Therefore, in order to obtain a high power LED, a plurality of LED elements are arranged in a series to form an LED array (for example, refer to Japanese Laid-open Patent Publication No. 2001-156331).
In application of headlamps for vehicles or the likes, an oblong LED array is required. However, increase in the number of LED elements is not preferable because a proportion of non-light-emitting regions between the elements increases. Thus a shape of each LED element in an LED array becomes an oblong.
FIG. 10A is a schematic plan view showing a conventional LED array 600, and FIG. 10B is a simplified cross sectional view of the LED array 600 shown in FIG. 10A.
Generally the conventional LED array 600 has four nitride semiconductor light emitting elements arranged and connected in a series on an insulating supporting substrate. In case of GaN-based white LED element, LED structures are formed on a sapphire substrate, a supporting substrate is adhered, the sapphire substrate is separated, and electrodes are formed.
Each LED element 601 has a GaN-based light emitting part 602 consisting of an n-type GaN layer 621, an active layer 622 and a p-type GaN layer 623, a p-electrode 612 formed on a back surface of the light emitting part 602, a wiring electrode (first wiring layer) 611 arranged on a right short side of the light emitting part 602 with a predetermined interval in parallel to the short side, and wiring electrodes (second wiring layers) 608 arranged on a surface of the light emitting part 602 in parallel to a long side of the light emitting part 602 and connecting the n-type GaN layer 621 with the wiring electrodes 611. The LED elements 601 adjacent horizontally (in a longitudinal direction of the LED elements 601) are connected with each other by forming the wiring electrode 611 of one (left-side) LED element 601 on the p-electrode 612 of the adjacent (right-side) LED element 601 in order to connect the n-type GaN layer 621 of the left-side element with the p-type GaN layer 623 of the right-side element.
A phosphor layer 631 seals the plurality of the LED elements 601 mounted on a substrate 630. For example, when the LED elements 601 are blue LED elements, a white LED array 600 can be fabricated by a combination of the blue LED elements and yellow phosphor. In this case, the yellow phosphor is added to transparent resin in advance, and the LED elements 601 are sealed by the transparent resin added with the phosphor.
Moreover, hatching of the light emitting part 602 in FIG. 10A indicates brightness distribution wherein increase in density of hatching indicates increase in brightness.
When the wiring electrode 611 is arranged in parallel to the short side of the LED element 601 and the wiring electrodes 608 on the n-type GaN layer 621 are arranged in parallel to the long side of the LED element 601, a length of the wiring electrode 608, for example, with a width of about 10 μm becomes long and its wiring resistance becomes large. Therefore, an injection current decreases from the right power supply side to the left side and it generates uneven brightness distribution.
Moreover, because the wiring electrode 611 with a width of about 40 μm is disposed between the LED elements 601, the interval between the LED elements 601 becomes wide and the brightness decreases; therefore, uneven brightness distribution is generated between the central and the peripheral areas of the element. If a headlamp or the likes is manufactured with the LED array 600 consisting of the above-described conventional LED elements 601, the uneven brightness is generated in a projection image.
FIG. 10C and FIG. 10D are diagrams showing the brightness distributions of the LED array 600 along the line e-f in FIG. 10A. FIG. 10C shows the brightness distribution along the line e-f of the LED array 600 without the phosphor layer 631 as a blue LED array, and FIG. 10D shows the brightness distribution along the line e-f of the LED array 600 with the phosphor layer 631 as a white LED array.
As shown in FIG. 10 C, without the phosphor layer 631 when blue light is emitted, a surface of the conventional element has a flat brightness distribution. However, after making the blue LED element emit white light by forming the phosphor layer 631 on the blue LED element having the flat brightness distribution, the brightness (maximum brightness) in the vertical center (center of the direction H in the drawing) becomes about 1.2 to 1.67 times of the brightness (reference brightness) at the edges. Therefore, the brightness distribution (lambertian distribution) in which the brightness gradually decrease from the center to the edge. By using this type of LED array 600 to compose a headlamp or the likes, uneven brightness distribution is generated in a projection image.